The semiconductor lasers are classified into the following three kinds of semiconductor lasers according to the combination of the cavity direction and the laser beam emitting direction. A first semiconductor laser is a horizontal cavity edge emitting laser, a second semiconductor laser is a vertical cavity surface emitting laser, and a third semiconductor laser is a horizontal cavity surface emitting laser. The first horizontal cavity edge emitting laser is designed in such a manner that an optical waveguide is formed in a horizontal direction within a substrate surface, and a laser beam is emitted from a facet obtained by dividing the substrate by cleaving. The above laser configuration is suitable to obtain a high output power and is most generally spread since the cavity length can be extended to about several hundreds. However, in the case of using this configuration, it is necessary to cleave the semiconductor substrate during a fabrication process. For that reason, the fabrication process and testing cannot be conducted without cleaving the wafer, resulting in such a demerit that the manufacture costs are increased.
The second vertical cavity surface emitting laser has such a configuration in which a cavity is formed in a direction perpendicular to the semiconductor substrate. For that reason, since it is unnecessary to cleave the substrate in order to form the cavity, there is an advantage that the fabrication and inspection of the laser can be conducted by the full wafer as it is, and the manufacture costs can be suppressed lowly. However, this configuration suffers from such a problem that the cavity length is very short because the cavity length is determined according to the crystal growth film thickness, and it is essentially difficult to obtain a high output power. On the contrary, the third horizontal cavity surface emitting laser has a laser configuration with the excellent points of the above two lasers. In this configuration, a cavity is formed in a horizontal direction on the substrate surface, and a mirror that is inclined by 45° for emitting the laser beam from a front surface or a rear surface of the substrate are integrated together. The present invention relates to a horizontal cavity surface emitting laser. The structural example of the horizontal cavity surface emitting laser will be described with reference to FIGS. 1A to 1D. FIG. 1A is a perspective view showing the cross section of the device, FIG. 1B is a cross sectional view showing the optical axial direction of the device, FIG. 1C is a diagram showing a lower surface of the device, and FIG. 1D is a cross sectional view perpendicular to the optical axis. The configuration of the device will be described hereinafter assuming that a surface on which the semiconductor substrate is placed is an x-y plane, a normal direction to the semiconductor substrate surface is a z-axial direction, and the optical axis direction of the laser cavity is an x-axial direction in order to three-dimensionally show the configuration of the laser.
This device is formed on an n-type InP substrate 11. A light is generated by injecting currents into an InGaAsP active layer 14 from an n-type electrode 12 of the substrate rear surface and a p-type electrode 13 of the substrate front surface. The generated beam is confined to an optical confinement configuration consisting of a p-type InP cladding layer 15, the active layer 14, and the n-type InP substrate 11 in the z-direction. Also, the beam is confined to an optical confinement configuration consisting of a semi-insulated InP layer 16, the active layer 14, and the semi-insulated InP layer 16 in the y-direction. In this way, the beam that is confined in the y-direction and the z-direction is propagated in the x-axial direction. A grating 17 that periodically changes the refractive index is formed in the x-axial direction along which the light is propagated. The light is fed back by the grating 17, and lasing is conduced. This laser is a so-called distributed feedback (DFB) laser. The laser beam thus generated is totally reflected by the mirror 18 that is formed by etching on one end of the waveguide with an angle of 45°, and then guided in the substrate rear surface direction. An anti-reflective coating film 19 is formed on a portion of the substrate rear surface which faces the 45° mirror, and the laser beam is emitted from the substrate rear surface.
In the horizontal cavity surface emitting laser with the above configuration, since the cavity is formed within the substrate plane, the cavity length can be extended, thereby making it easy to obtain a high output power. Also, since a light is emitted in a direction perpendicular to the substrate surface, it is possible to fabricate and test the laser in the full wafer process, and the costs expended for the manufacture are lowly suppressed. As a known example of the conventional horizontal cavity surface emitting laser, JP-A No. 2004-235182 discloses a horizontal cavity surface emitting laser having a distributed Bragg reflector that is formed on the semiconductor substrate, an optical waveguide layer that is formed on the reflector, and a reflector that is formed on an end of the optical waveguide layer with an angle of 45°. Also, as another known example, JP-A No. 2007-5594 discloses a horizontal cavity surface emitting laser having an active region of 10 to 100 μm, a distributed Bragg reflector, and an oblique mirror. Also, as a third known example, “IEEE Photonics Technology Letters” Vol. 3, No. 9, p. 776 reports the room temperature continuous-wave lasing characteristics of a horizontal cavity surface emitting laser having an optical waveguide with an InGaAsP active layer that is formed on an InP substrate, a reflector that is formed on an end of the optical waveguide with an angle of 45°, and a circular lens that is formed at a position that faces the reflector.